In recent years, there is a rapidly increasing demand for large flat screen displays. To meet this demand, there have been efforts to increase the screen size of plasma displays, liquid crystal displays and organic EL displays.
It is not practical, however, to install displays above a certain size in houses, assembly halls, conference rooms and the like because this hinders the use of these spaces for other purposes.
For this reason, in order to watch DVDs or Blue-ray Discs on a large screen above a certain size or to output data from PC screens, projection display devices such as projectors are often used. Projectors are compact enough to be portable and have a screen that can be rolled up and stored compactly.
Projectors, however, have a disadvantage that it is difficult to display bright images on the screen. As another disadvantage, it is troublesome to focus the lens every time they are set up.
To overcome these disadvantages, there have been suggestions to make large flat-screen direct-view display devices having a liquid crystal display or an organic EL display flexible enough to be rolled up. For example, Japanese Patent Unexamined Publication No. H07-28395 discloses a device which accommodates a bendable liquid crystal display in a display housing. Japanese Patent Unexamined Publication No. 2002-328625 discloses a display device which includes a screen-like display part that can be rolled up for storage.
An organic EL element used in an organic EL display has the following preferable features: self-luminousness, excellent visibility, rapid response, lightweightness, slimness, and the ability to be driven at a low voltage of several volts or less. Display devices using such an organic EL element have a flexible structure because a display function layer and a layer including an electric function part for working the display function layer are both made thin.
In general, an organic EL display includes as separate devices peripheral circuits such as a power supply, a drive device and a control device, and an acoustic device such as a speaker. For example, in Japanese Patent Unexamined Publication No. 2002-162652, the display device and these devices are connected to each other using an electric wire, a flexible circuit board, or the like.
However, when a display device using a self-luminous element such as an organic EL display with a large screen above a certain size like 100 in or more is designed to be rolled up, the high flexibility causes the following problems when the screen is set up.
For one thing, when a sheet-like large image display device using an organic EL is set up, the image display part is swayed by the wind from an air conditioner or shaken by vibration from the acoustic device such as a speaker, making it harder to provide clear images.
For another thing, it is difficult to property set up the display device itself. For example, when a sheet-like image display device is placed in the horizontal direction, the sheet-like display part tends to sag in the center.
Such swaying and sagging should be absolutely eliminated in a large-screen display device using a self-luminous element such as an organic EL display that can be rolled up for storage because of the following reason.
In the case of a projector, swaying or sagging of the screen never alters the position of the projector main body and only causes images to be displayed in an enlarged or reduced size according to the swaying. This allows the audience to continue to see images unless the screen is inclined too much.
In contrast, in the case of a large-screen direct-view display device including a self-luminous element such as an organic EL display, the images on the display part are generated by the display part itself, unlike the projector. Therefore, if the display part sways, the images and texts shown on the display part can be greatly changed in position. This means that the swaying of the screen directly leads to difficulty in displaying clear images. Furthermore, images on the area of the screen that is sagged in accordance with the direction to place the display device are displayed in a curved shape.
For yet another thing, in a large-screen display device using a self-luminous element such as an organic EL display, images to be displayed on the display part are generated by the own mechanism of the display part. This may be highly likely to cause trouble when the display part sways and hits a wall or a stationary object around it, or sags and bends to damage pixels while being set up.
In the case of the projector, external devices are connected to the projector main body, and the screen just displays images projected from the projector main body. Therefore, the swaying of the screen never triggers trouble in the projector main body or the external devices. In contrast, the large-screen display device using a self-luminous element such as an organic EL display is directly connected with the external devices using a distribution cable. Therefore, the swaying of the display device may cause the distribution cable connected with the external devices to be yanked, thereby pulling the wiring out or causing the external devices yanked along with the distribution cable to fall off the shelf.
This is why large-screen display devices using a self-luminous element that can be rolled up for storage require countermeasures against swaying and sagging. The countermeasures should be something that does not hinder the function of the display part, which is, to be rolled up for storage.
For example, Japanese Patent Unexamined Publication No. H07-28395 suggests a display device which has display fixing frames for fixing both ends of a bendable liquid crystal display. However, in a lightweight and flexible sheet-like image display device using a self-luminous element such as an organic EL element, the aforementioned problem cannot be fully solved only by the mechanism of stretching the display fixing frames to the display ends. More specifically, the mere provision of the display fixing frames on the ends of the display cannot sufficiently reduce vibration or sagging at the center of the display part that is not supported by the display fixing frames.